kvm: use separate MemoryListeners for memory and I/O

The construct

   if (address_space == get_system_memory()) {
       // memory thing
   } else {
       // io thing
   }

fails if we have more than two address spaces.  Use a separate listener
for memory and I/O, and utilize MemoryListener's address space filtering to
fix this.

Signed-off-by: Avi Kivity <avi@redhat.com>